The innovations disclosed herein pertain to interferometer systems, and more particularly, but not exclusively, to fiber-optic interferometer systems, such as, for example, systems used in security, surveillance or monitoring applications. Some disclosed interferometer systems relate to detecting and locating disturbances (e.g., a disturbance to a secure perimeter, such as a “cut” on a fence, a leak from a pipeline, a change in structural integrity of a building, a disturbance to a communication line, a change in operation of a conveyor belt, an impact on a surface or acoustical noise, among others) with one or more passive sensors.
Earlier attempts at using interferometer-based systems to detect disturbances have met with varying degrees of success. For example, a Mach-Zehnder interferometer can detect a phase-shift between two beams of light split from a single collimated beam. When two respective optical path lengths differ, the respective beams typically will be out of phase and a Mach-Zehnder interferometer can detect such a phase difference. Thus, a Mach-Zehnder interferometer can detect a change in relative optical-path lengths, such as can occur when one of a pair of optical conduits carrying optical signals is perturbed differently than the other of the pair. Nonetheless, a Mach-Zehnder interferometer alone cannot provide the location of such a disturbance or the magnitude of the difference in path lengths.
Systems including interferometers configured to detect a disturbance have been proposed. For example, U.S. Pat. No. 6,778,717 discloses a method that includes launching light in opposite directions through a single Mach-Zehnder interferometer to form counter-propagating optical signals that can be modified by a perturbation of the interferometer (also referred to as a “disturbance” or an “event”). The '717 patent discloses that the position of such an event can be determined by substantially continuously and simultaneously monitoring respective modified counter-propagating optical signals and determining the time difference between the separately detected modified signals. The disclosure in the '717 patent is incorporated herein in its entirety by reference.
U.S. Pat. Nos. 7,499,176 and 7,499,177 disclose improvements to the technology disclosed in U.S. Pat. No. 6,778,717. The '176 and '177 patents are directed to methods and apparatus for actively controlling polarization states of counter-propagating optical signals passing through a Mach-Zehnder interferometer so as to match phase and/or amplitude between the counter-propagating signals. With the technology disclosed in U.S. Pat. No. 6,778,717, substantially matched polarization states are required to correlate the output corresponding to each of the counter-propagating signals to the other respective signal outputs. Such an interferometer is shown schematically in FIG. 1. The disclosure in the '176 and '177 patents are incorporated herein in their entirety by reference.
To actively control the polarization states of the counter-propagating signals, a polarization controller is needed at each input of the Mach-Zehnder interferometer's light paths. Such polarization controllers that provide matched polarization states are costly. Also, since at least some polarization controllers are configured to tune polarization states so that observed output signals have no amplitude- or phase-shift between them, when a sensor is momentarily perturbed and a polarization-induced phase-shift between counter-propagating signals is thereby introduced, a significant amount of time can elapse after the perturbation and before the polarization controllers have suitably matched polarization states to detect a subsequent perturbation. Therefore, a significant amount of time can elapse before a subsequent disturbance can be detected and located accurately.
Therefore, systems as disclosed in U.S. Pat. Nos. 7,499,176, 7,499,177 and 6,778,717 suffer serious deficiencies. For example, perimeter security systems incorporating such systems can be bypassed by introducing a diversionary disturbance at one location and subsequently crossing a monitored perimeter at another location some distance away from the location of the diversionary disturbance while the polarization controllers are being “reset” (e.g., are attempting to re-match polarization states).
Other approaches for detecting disturbances have also been proposed. For example, U.S. Pat. No. 7,514,670, describes a low-cost system having a distributed plurality of sensitive “zones.” In particular, the '670 patent discloses a system having an optical conduit configured to convey light past a plurality of sensitive regions and to split off a fraction of light into each of the sensitive regions. Each of the sensitive regions comprises, for example, an interferometer configured to detect a disturbance.
Since a portion of an incoming beam of light is diverted into each sensitive region (or zone), such a system has practical limitations on the number of zones that are possible when using a given light source. As a result of being limited to a particular number of zones, there is also a practical limitation on the length of a perimeter that can be monitored with such a system.
The '670 patent discloses that the presence of a disturbance can be isolated to a particular zone, so such a system can generally identify the location of a disturbance. However, such a zone can span a relatively large distance, which might not provide a desired spatial resolution for many security applications. For example, some security applications require that a system identify the location of a disturbance to within several (e.g., less than about ten) meters (such as, for example, to within between about 3 meters and about 5 meters).
Thus, a need remains for simpler and less costly systems for accurately detecting the existence, position or magnitude of a disturbance. There also remains the need for systems that provide these advantages over a distance of many kilometers. There also remains the need for systems that can detect the existence, position or magnitude of a subsequent disturbance within fewer than about 3 seconds of an initial event or disturbance.